Average Follow-up Time DetailBaseline evaluation was between January 1994 and July 2001.

Exposure Detail"We assessed participation in physical activities with questions adapted24 from the 1985 Health Interview Survey.25 The activities were walking for exercise, gardening or yardwork, calisthenics or general exercise, bicycle riding, and swimming or water exercise. Persons were asked if they had participated in each activity in the last 2 weeks, and if so, the number of occasions and average minutes per occasion. Minutes in each activity were summed and divided by 120 to yield a composite measure of participation in physical activity expressed as hours per week. Because results were unchanged when activities were weighted by the estimated energy expended,26 we used total weekly hours in all analyses. Due to its skewed distribution, we treated it as a categorical variable in the main analysis with people grouped into quartiles."The least active group was the reference category.

Screening and Diagnosis DetailScreening Method:

CERAD

Consortium to Establish a Registry for Alzheimer's Disease (Morris 1989)

"At baseline, each person had a uniform structured evaluation that was repeated annually by examiners blinded to previously collected data. The evaluation has been previously described.9-11 [The procedure followed was that recommended by CERAD.] It included a medical history, neurological examination, assessment of cognitive function, and review of brain scan when available. On the basis of this evaluation, a board-certified neurologist diagnosed AD and other common conditions affecting cognitive or physical function (eg, stroke). The diagnosis of AD followed the criteria of the joint working group of the National Institute of Neurological and Communicative Disorders and Stroke and the Alzheimer's Disease and Related Disorders Association (NINCDS/ADRDA).12 These criteria require a history of cognitive decline and impairment in memory and at least 1 other cognitive domain. Some persons who met these criteria had another condition impairing cognition (termed "possible" AD in the NINCDS/ADRDA system). Because exclusion of this subgroup did not affect results, it is included in all analyses reported in this article."

"At baseline, persons spent a median of 3.5 hours per week in physical activities (interquartile range, 0.5-7.0 ; mean, 5.7; SD, 8.3 hours). Because of the skewed distribution, we divided physical activity time into quartiles and contrasted those in the lowest quartile with each of the remaining quartiles in a proportional hazards model that controlled for age, sex, and education...."Similar results were obtained when the analysis was repeated with physical activity treated as a continuous variable (HR, 1.00; 95% CI, 0.97-1.02). Results were also comparable when cognitive activity was added to the model. In random-effects models that controlled for demographic variables, physical activity was not related to decline in global or specific measures of cognitive function."

The least active group was the reference category in the categorical analysis.